4. Design of aquaponic units · 2021. 4. 13. · 36 Small-scale aquaponic food production – Integrated fish and plant farming FIGURE 4.3 Taro (Colocasia esculenta) plants growing

35

4. Design of aquaponic units

This chapter discusses the theory and design of aquaponic systems. There are many design aspects to take into consideration, as virtually all environmental and biological factors will have an impact on the aquaponic ecosystem. The aim of this chapter is to present these aspects in the most accessible way and to provide a thorough explanation of each component within an aquaponic unit.

Section 4.1 discusses the factors to consider when selecting a site for an aquaponic unit, including access to sunlight, wind and rain exposure, average temperature and others. Section 4.2 discusses the general aquaponic components essential for any method of aquaponics, including the fish tank, water and air pumps, the biofilter, the plant growing method and associated plumbing materials. The hydroponic component is then discussed in further detail, focusing on the three most common methods used in aquaponics: the media bed method (Figures 4.1–4.5); the nutrient film technique (NFT) method (Figures 4.6–4.9); and the deep water culture (DWC) method (Figures 4.10–4.13).

A specific section then presents a particular type of DWC with low stocking density. A final summary table of each method is provided in order to compare and contrast these three methods.

This chapter is intended only to explain the essential unit components and different methods of aquaponics. For more information regarding the sizing and design ratios for different unit components, please see Chapter 8, which provides the more detailed information, figures and design plans needed to actually design and construct small-scale aquaponic units. In addition, Appendix 8 gives a full step-by-step guide to building a small-scale version of the three methods explained in this chapter using materials widely available.

FIGURE 4.1Illustration of a small media bed unit

Fish tank Sump Plant growing area

Method Abbreviation Other names Name of planting area Section

Deep water culture DWC floating raft canal, trough 4.3

Nutrient film technique NFT pipe, gutter 4.4

Media bed n/a particulate bed, tray 4.5

Small-scale aquaponic food production – Integrated fish and plant farming36

FIGURE 4.3Taro (Colocasia esculenta) plants growing in a

semi-commercial media bed unit constructed in wood and lined with polyethylene liner

FIGURE 4.4Lush vegetable growth in a backyard media

bed unit

FIGURE 4.2Example of a newly assembled media bed unit using intermediate bulk containers

FIGURE 4.5A media bed unit planted with chili pepper

(Capsicum spp.)

37

FIGURE 4.9A nutrient film technique unit using vertical space

Design of aquaponic units

FIGURE 4.6Illustration of a small nutrient film technique unit

Fish tank Filters Plant growing area

FIGURE 4.7Parsley (Petroselinum sp.) growing in a small

nutrient film technique unit

FIGURE 4.8Farmers tending young tomato plants in a

nutrient film technique unit. Net cups are made from recycled plastic bottles with holes in the

bottom


4.1 SITE SELECTIONSite selection is an important aspect that must be considered before installing an aquaponic unit. This section generally refers to aquaponic units built outdoors without a greenhouse. However, there are brief comments about greenhouses and shading net structures for larger units. It is important to remember that some of the system’s components, especially the water and stone media, are heavy and hard to move, so it is worth building the system in its final location. Selected sites should be on a surface

FIGURE 4.10Illustration of a small deep water culture unit


FIGURE 4.11Lettuce plants growing in a deep water

culture unit

FIGURE 4.12Multiple varieties of lettuce plants growing in a

deep water culture unit

FIGURE 4.13Roots of curly kale (Brassica sp.) growing in a

deep water culture unit

39Design of aquaponic units

that is stable and level, in an area that is protected from severe weather but exposed to substantial sunlight.

4.1.1 StabilityBe sure to choose a site that is stable and level. Some of the major components of an aquaponic system are heavy, leading to the potential risk of the legs of the system sinking into the ground. This can lead to disrupted water flow, flooding or catastrophic collapse. Find the most level and solid ground available. Concrete slabs are suitable, but do not allow any components to be buried, which can lead to tripping hazards. If the system is built on soil, it is useful to grade the soil and put down material to mitigate weeds. In addition, place concrete or cement blocks under the legs of the grow beds to improve stability. Stone chips are often used to level and stabilize soil locations. Moreover, it is important to place the fish tanks on a base; this will help to provide stability, protect the tank, allow for plumbing and drains on the tank bottom, and thermally isolate it from the ground.

4.1.2 Exposure to wind, rain and snowExtreme environmental conditions can stress plants and destroy structures (Figure 4.14). Strong prevailing winds can have a considerable negative impact on plant production and can cause damage to stems and reproductive parts. In addition, strong rain can harm the plants and damage unprotected electrical sockets. Large amounts of rain can dilute the nutrient-rich water, and can flood a system if no overflow mechanism is integrated into the unit. Snow causes the same problems as heavy rain, with the added threat of cold damage. It is recommended that the system be located in a wind-protected zone. If heavy rains are common, it may be worth protecting the system with a plastic-lined hoop house, although this may not be necessary in all locations.

4.1.3 Exposure to sunlight and shadeSunlight is critical for plants, and as such, the plants need to receive the optimum amount of sunlight during the day. Most of the common plants for aquaponics grow well in full sun conditions; however, if the sunlight is too intense, a simple shade structure can be installed over the grow beds. Some light-sensitive plants, including lettuce, salad greens and some cabbages, will bolt in too much sun, go to seed and become bitter and unpalatable. Other tropical plants adapted to the jungle floor such as turmeric and certain ornamentals can exhibit leaf burn when exposed to excessive sun, and they do better with some shade. On the other hand, with insufficient sunlight, some plants can have slow growth rates. This situation can be avoided by placing the aquaponic unit in a sunny location. If a shady area is the only location available, it is recommended that shade-tolerant species be planted.

Systems should be designed to take advantage of the sun travelling from east to west through the sky. Generally, the grow beds should be spatially arranged such that the longest side is on a north–south axis. This makes the most efficient use of the sun during the day. Alternatively, if less light is preferable, orient the beds, pipes and canals following the east–west axis. Also consider where and when there are shadows that cross

FIGURE 4.14Deep water culture system damaged by snow


the chosen site. Be careful in the arrangement of plants such that they do not inadvertently shade one another. However, it is possible to use tall, sun-loving plants to shade low, light-sensitive plants from intense afternoon sun by placing the tall plants to the west or by alternating the two in a scattered distribution.

Unlike the plants, the fish do not need direct sunlight. In fact, it is important for the fish tanks to be in the shade. Normally, the fish tanks are covered with a removable shading material that is placed on top of the tank (Figure 4.15). However, where possible, it is better to isolate the fish tanks using a separate shading structure. This will prevent algae growth (see Chapter 3) and will help to maintain a stable water temperature during the day. It is also worth preventing leaves and organic debris from entering the fish tanks, as the decaying leaf matter can stain the water, affect water chemistry and clog pipes. Either locate the system away from overhanging vegetation or keep the tank covered with a

screen. Moreover, fish tanks are vulnerable to predators. Using shade netting, tarps or other screening over the fish tanks will prevent all of these threats.

4.1.4 Utilities, fences and ease of accessIn site selection, it is important to consider the availability of utilities. Electric outlets are needed for water and air pumps. These outlets should be shielded from water and equipped with a residual-current device (RCD) to reduce the risk of electrical shock; RCD adaptors can be purchased from standard hardware stores. Moreover, the water source should be easily accessible, whether it is municipal water or rain collection units. Similarly, consider where any effluent from the system would go. Although extremely water efficient, aquaponic systems occasionally require water changes, and filters and clarifiers need to be rinsed. It is convenient to have some soil plants located nearby that would benefit from this water. The system should be located where it is easy for daily access because frequent monitoring and daily feeding are required. Finally, consider if it is necessary to fence the entire section. Fences are sometimes required to prevent theft and vandalism, animal pests and for some food safety regulations.

4.1.5 Special considerations: rooftop aquaponicsFlat rooftops are often suitable sites for aquaponics because they are level, stable, exposed to sunlight and are not already used for agriculture (Figures 4.16–4.18). However, when building a system on a rooftop it is crucial to consider the weight of the system, and whether or not the roof is capable of supporting it. It is essential to consult with an architect or civil engineer before building a rooftop system. In addition, be sure that materials can be transported both safely and effectively to the rooftop site.

FIGURE 4.15Shade material (blue) filtering sunlight in the

fish tank

FIGURE 4.16A small media bed unit on a rooftop


4.1.6 Greenhouses and shading net structuresGreenhouses are not essential for small-scale aquaponic units, yet they may be useful in extending the growing season in some regions (Figures 4.19 and 4.20). This is particularly true in temperate and other cooler regions around the world, as greenhouses can be used to maintain a warm water temperature during the cold months, thereby allowing year-round production.

A greenhouse is a metal, wood or plastic frame structure that is covered by transparent nylon, plastic or glass. The purpose of this structure is to allow sunlight (solar radiation) to enter the greenhouse and then trap it so it begins heating the air inside the greenhouse. As the sun begins to set, the heat is retained in the greenhouse by the roof and walls, allowing for a warmer and more stable air temperature during a 24-hour period. Greenhouses provide general environmental protection from wind, snow and heavy rain. Greenhouses extend the growing season by retaining ambient solar heat, but can also be heated from within. Greenhouses can keep away animals and other pests, and serve as some security against theft. Greenhouses are comfortable to work in during colder seasons, and provide the grower with protection from the weather. Greenhouse frames can be used to support climbing plants or to hang shade material. Together, these advantages of a greenhouse result in higher productivity and in an extended cropping season.

However, these benefits need to be balanced against the drawbacks of greenhouses. The initial capital costs for a greenhouse can be high depending on the degree of technology and sophistication desired. Greenhouses also require additional operating costs because fans are needed to create air circulation to prevent overheating and overly humid conditions. Some

FIGURE 4.17Multiple aquaponic systems on a rooftop

FIGURE 4.18Variety of vegetables growing on a rooftop in

nutrient film technique systems

FIGURE 4.19Small aquaponic units in a greenhouse

FIGURE 4.20Newly assembled aquaponic units in a greenhouse


diseases and insect pests are more common in greenhouses and need to be managed accordingly (i.e. use of insect nets on doors and windows), although the confined environment can favour the use of certain pest controls.

In some tropical regions, net houses are more appropriate than conventional greenhouses covered with polyethylene plastic or glass (Figure 4.21). This is because the hot climates in the tropics or subtropics raise the need for better ventilation to avoid high temperatures and humidity. Net houses consist of a frame over the grow beds that is covered with mesh netting along the four walls and a plastic roof over the top. The plastic roof is particularly important

to prevent rain from entering, especially in areas with intense rainy seasons, as units could overflow in a matter of days. Net houses are used to remove the threat of many noxious pests associated with the tropics, as well as birds and larger animals. The ideal mesh size for the four walls depends on the local pests. For large insects, the mesh size should be 0.5 mm. For smaller ones, which are often vectors of viral diseases, the mesh size should be thicker (i.e. mesh 50). Net houses can provide some shade if the sunlight is too intense. Common shade materials vary from 25 to 60 percent sunblock.

4.2 ESSENTIAL COMPONENTS OF AN AqUAPONIC UNITAll aquaponic systems share several common and essential components. These include: a fish tank, a mechanical filter, a biofilter, and hydroponic containers. All systems use energy to circulate water through pipes and plumbing while aerating the water. As introduced above, there are three main designs of the plant growing areas including: grow beds, grow pipes and grow canals. This section discusses the mandatory components, including the fish tanks, mechanical filter, biofilter, plumbing and pumps. The following sections are dedicated to the separate hydroponic techniques, and a comparison is made to determine the most appropriate combination of techniques for different circumstances.

4.2.1 Fish tankFish tanks are a crucial component in every unit. As such, fish tanks can account for up to 20 percent of the entire cost of an aquaponic unit. Fish require certain conditions in order to survive and thrive, and therefore the fish tank should be chosen wisely. There are several important aspects to consider, including the shape, material and colour.

Tank shape Although any shape of fish tank will work, round tanks with flat bottoms are recommended. The round shape allows water to circulate uniformly and transports solid wastes towards the centre of the tank by centripetal force. Square tanks with flat bottoms are perfectly acceptable, but require more active solid-waste removal. Tank shape greatly affects water circulation, and it is quite risky to have a tank with poor circulation. Artistically shaped tanks with non-geometric shapes with many curves and bends can create dead spots in the water with no circulation. These areas can gather wastes and create anoxic, dangerous conditions for the fish. If an odd-shaped tank is to be used, it may be necessary to add water pumps or air pumps to ensure proper circulation and remove the solids. It is important to choose a tank to fit the characteristics of the aquatic species reared because many species of bottom dwelling fish show better growth and less stress with adequate horizontal space.

FIGURE 4.21Net house structure to house a small aquaponic unit


Material Either strong inert plastic or fibreglass is recommended because of their durability and long life span. Metal is not possible because of rust. Plastic and fibreglass are convenient to install (also for plumbing) and are fairly light and manoeuvrable. Animal-watering troughs are commonly used, as they tend to be cheap. If using plastic containers, make sure that they are UV-resistant because direct sunlight can destroy plastic. In general, low-density polyethylene (LDPE) tanks are preferable because of their high resistance and food-grade characteristics. Indeed, LDPE is the most commonly used material for water storage tanks for civil uses. Another option is an in-ground pond. Natural ponds are very difficult to manage for aquaponics because the natural biological processes, already occurring within the substrate and mud at the bottom, can be hard to manipulate and the nutrients are often already used by aquatic plants. Cement or plastic-lined ponds are much more acceptable, and can be an inexpensive option. In-ground ponds can make plumbing operations difficult, and the plumbing design should be carefully considered before embarking on this option. One of the simplest fish tanks is a hole dug in the ground, lined with bricks or cinderblocks, and then lined with a waterproof liner such as polyethylene plastic. Other options include second-hand containers, such as bathtubs, barrels or intermediate bulk containers (IBCs). It is very important to make sure the container has not been used previously to store toxic material. Contaminants, such as solvent-borne chemicals, will have penetrated into the porous plastic itself and are impossible to remove with washing. Thus, choose used containers carefully, and know the seller if possible.

Colour White or other light colours are strongly advised as they allow easier viewing of the fish in order to easily check behaviour and the amount of waste settled at the bottom of the tank (Figures 4.22–4.24). White tanks will also reflect sunlight and keep the water cool. Alternatively, the outside of darker coloured tanks can be painted white. In very hot or cold areas, it may be necessary to further thermally insulate the tanks. Covers and shading All fish tanks should be covered. The shade covers prevent algal growth. In addition, the covers prevent fish from jumping out (often occurs with newly added fish or if water quality is sub-optimal), prevent leaves and debris from entering, and prevent predators such as cats and birds from attacking the fish. Often, agricultural shading nets that block 80–90 percent of sunlight are used. The shade cloth can be attached to a simple wooden frame to provide weight and make the cover easy to remove.

Failsafe and redundancyDo not let the fish tank lose its water; fish will die if the fish tank accidentally drains. Although some accidents are unavoidable (e.g. a tree falling on the tank), most catastrophic fish kills are the result of human error. Ensure that

FIGURE 4.22A 1 000 litre fish tank made from a white

polyethylene drum

FIGURE 4.23young fish in a cylindrical aquaponic tank. Return

line (top) and bottom drain clearly visible


there is no way for the tank to drain without a deliberate choice by the operator. If the water pump is located in the fish tank, be sure to lift the pump off the bottom so that the tank can never be pumped dry. Use a standpipe inside the tank to guarantee a minimum water level. This is discussed further in Section 4.2.6. 4.2.2 Filtration – mechanical and biological Mechanical filtrationFor RASs, mechanical filtration is arguably the most important aspect of the design. Mechanical filtration is the separation and removal of solid and suspended fish waste from fish tanks. It is essential to remove these wastes for the health of the system, because harmful gases are released by anaerobic bacteria if solid waste is left to decompose inside the fish tanks. Moreover, the wastes can clog systems and disrupt water flow, causing anoxic conditions to the plant roots. Small-scale aquaponics typically has lower stocking densities than the intensive RAS

methods for which these mechanical filters were originally designed, but some level of mechanical filtration is essential for healthy aquaponic tanks, regardless of the type of hydroponic method used.

There are several types of mechanical filters. The simplest method is a screen or filter located between the fish tank and the grow bed. This screen catches solid wastes, and needs to be rinsed often. Similarly, water leaving the fish tank can pass through a small container of particulate material, separate from the media bed; this container is easier to rinse periodically. These methods are valid for some small-scale aquaponic units, but are insufficient in larger systems with more fish where the amount of solid waste is relevant. There are many types of mechanical filters, including sedimentation tanks, radial-flow clarifiers, sand or bead filters and baffle filters; each of them can be used according to the amount of solid wastes that needs to be removed. However, as this publication focuses on small-scale aquaponics, clarifiers, or mechanical separators, are the most appropriate filters. Clarifiers, in general, can remove up to 60 percent of the total removable solids. For further information on different methods of mechanical filtration, please refer to the further reading section at the end of this publication.

Mechanical separators (clarifiers)A clarifier is a dedicated vessel that uses the properties of water to separate particles. Generally, water that is moving slower is unable to carry as many particles as water that is flowing faster. Therefore, the clarifier is constructed in such a way as to speed up and slow down the water so that the particles concentrate on the bottom and can be removed. In a swirl clarifier, the water from the fish tank enters near the lower-middle of the clarifier through a pipe. This pipe is positioned tangentially to the container thereby forcing the water to swirl in a circular motion inside the container. The centripetal force created by the circular motion of the water forces the solid waste in the water to the centre and bottom of the container, because the water in the centre of the vortex is slower than that on the outside. Once this waste is collected on the bottom, a pipe attached to the bottom of the container can be periodically opened, allowing the solid waste to flush out of the container. The clarified water exits the

FIGURE 4.24Two large (1 000 litre each) rectangular fish tanks

holding separate cohorts of juvenile fish


clarifier at the top, through a large slotted outlet pipe covered with a secondary mesh filter, and flows into the biofilter or into the media beds. Figures 4.25–4.27 show examples of simple mechanical separators for small to large units. The solid wastes trapped and removed contain nutrients and are very useful for the systems or for garden plants in general; mineralization of solid waste is discussed in the following section. A general guideline for small-scale units is to size the mechanical separator container to be about one-sixth the volume of the fish tank, but this depends on stocking density and the exact design. Appendix 8 contains detailed, step-by-step instruction on the construction of each part of these systems.

Adequate preliminary mechanical filtration is especially important for NFT and DWC units used to trap and remove solid waste. Without this preliminary process, solid and suspended waste will build up in the grow pipes and canals and will clog the root surfaces. Solid waste accumulation causes blockages in pumps and plumbing components. Finally, unfiltered wastes will also create hazardous anaerobic spots in the system. These anaerobic spots can harbour bacteria that produce hydrogen sulphide, a very toxic and lethal gas for fish, produced from fermentation of solid wastes, which can often be detected as a rotten egg smell.

BiofiltrationBiofiltration is the conversion of ammonia and nitrite into nitrate by living bacteria. Most fish waste is not filterable using a mechanical filter because the waste is dissolved directly in the water, and the size of these particles is too small to be mechanically removed. Therefore, in order to process this microscopic waste an aquaponic system uses microscopic bacteria. Biofiltration is essential in aquaponics because ammonia and nitrite are toxic even at low concentrations, while plants need the nitrates to grow. In an aquaponic unit, the biofilter is a deliberately installed component to house a majority of the living bacteria. Furthermore, the dynamic movement of water within a biofilter will break down very fine solids not captured by the clarifier, which further prevents waste build up on plant roots in NFT and DWC. However, some large aquaponic facilities following the design of the system developed at the University of the Virgin Islands do not use a separate biofilter as they mostly rely on the units’ wet surfaces, on plant roots and direct plant uptake to process ammonia. Separate biofiltration is unnecessary in the media bed technique because the grow beds themselves are perfect biofilters.

FIGURE 4.25Diagram of a mechanical solids separator

Water to biofilter

Water from fish tank

Swivel to darin sludge

FIGURE 4.26Picture of a mechanical solids separator

Water to biofilter

Water from fish tank

Open to drain sludge

FIGURE 4.27Diagram of a mechanical solids separator with baffles


The biofilter is designed to have a large surface area supplied with oxygenated water. The biofilter is installed between the mechanical filter and the hydroponic containers. The minimum volume of this biofilter container should be one-sixth that of the fish tank. Figure 4.28 shows an example of a biofilter for small-scale units.

One commonly used biofilter medium is Bioballs® a proprietary product available from aquaculture supply stores, although similar generic brands exist (Figure 4.29). These are designed to be an ideal biofilter material, because they are small, specially shaped plastic items that have a very large surface area for their volume (500–700 m²/m³). Other media can be used, including volcanic gravel, plastic bottle

caps, nylon shower poufs, netting, polyvinyl chloride (PVC) shavings and nylon scrub pads. Any biofilter needs to have a high ratio of surface area to volume, be inert and be easy to rinse. Bioballs® have almost double the surface area to volume ratio of volcanic gravel, and both have a higher ratio than plastic bottle caps. When using suboptimal biofilter material, it is important to fill the biofilter as much as possible, but even so the surface provided by the media may be not sufficient to ensure adequate biofiltration. It is always better to oversize the biofilter during the initial construction, but secondary biofilters can be added later if necessary. Biofilters occasionally need stirring or agitating to prevent clogging, and occasionally need rinsed if the solid waste has clogged them, creating anoxic zones. See Chapter 8 and Appendix 4 for further information on biofiltration size requirements for small-scale units.

Another required component for the biofilter is aeration. Nitrifying bacteria need adequate access to oxygen in order to oxidize the ammonia. One easy solution is to use an air pump, placing the air stones at the bottom of the container. This ensures that the bacteria have constantly high and stable DO concentrations. Air pumps also help break down any solid or suspended waste not captured by the mechanical separator by agitating and constantly moving the floating

Bioballs®. To further trap solids within the biofilter, it is also possible to insert a small cylindrical plastic bucket full of nylon netting (such as Perlon®), sponges or a net bag full of volcanic gravel at the inlet of the biofilter (Figure 4.30). The waste is trapped by this secondary mechanical filter, allowing the remaining water to flow down through small holes drilled at the bottom of the bucket into the biofilter container. The trapped waste is also subject to mineralization and bacterial degradation.

FIGURE 4.28 Diagram of a biofilter for small-scale nutrient film

technique and deep water culture units

Water from clarifier

Water to grow beds

Biofilter media

Pump

FIGURE 4.29Detail of plastic biofilter medium with large specific

surface area

FIGURE 4.30Details of biofilter showing the (a) additional

mechanical filtration and (b) the biofilter medium

b

a


MineralizationMineralization, in terms of aquaponics, refers to the way that solid wastes are processed and metabolized by bacteria into nutrients for plants. Solid wastes that are trapped by the mechanical filter contain nutrients; although processing these wastes is different from biofiltration and requires separate consideration. Retaining the solids within the overall system will add more nutrients back to the plants. Any waste that remains on the mechanical filters, within the biofilters or in the grow beds is subjected to some mineralization. Leaving the waste in place for longer allows more mineralization; longer residence time of the waste in the filters will lead to more mineralization and more nutrients being retained in the system. However, this same solid waste, if not properly managed and mineralized, will block water flow, consume oxygen and lead to anoxic conditions, which in turn lead to dangerous hydrogen sulphide gas production and denitrification. Some large systems therefore deliberately leave the solid waste within the filters, ensuring adequate water flow and oxygenation, so that a maximum of the nutrients is released. However, this method is impractical for small-scale NFT and DWC systems. If it is decided to deliberately mineralize these solids, there are simple ways to facilitate the bacterial breakdown in a separate container, simply storing these wastes in this separate container with adequate oxygenation using air stones. After an indefinite amount of time, the solid waste will have been consumed, metabolized and transformed by heterotrophic bacteria. At this point, the water can be decanted and re-added to the aquaponic system, and the remaining waste, which has decreased in volume, can be added to the soil.

Alternatively, these solid wastes can be separated, removed and added to any in-ground agriculture, garden or compost bin as a valuable fertilizer. However, losing these nutrients can cause deficiencies in the plants which may then require supplementation of nutrients (see Chapter 6) .

Using a media bed for a combination of mechanical and biological filtrationIt is also possible to use a media-filled bed for mechanical and biofiltration in NFT and DWC units (Figures 4.31 and 4.32). This can be important where it is not possible to obtain the materials needed for a swirl separator and/or separate biofilter. Although more fully discussed in Chapter 8, here it is sufficient to say that for every 200 g of fish feed per day the biofilter needs to be 300 litres in volume. This small gravel would provide adequate biofiltration for about 20 kg of fish. Although this media bed would provide adequate biofiltration for an NFT or DWC unit as well as capturing and retaining solid wastes, an additional solids capture device placed into the bed is sometimes recommended in order to prevent the media bed

FIGURE 4.31Small-scale media bed unit using a screen for

additional mechanical filtration

FIGURE 4.32A media bed unit used for filtration in a deep

water culture system


from clogging with fish solids. The bed will need rinsing periodically to remove solid wastes.

In summary, some level of filtration is essential to all aquaponics, although fish stocking density and system design determines how much filtration is necessary. Mechanical filters separate solid wastes to prevent toxic build up, and biofiltration converts dissolved nitrogenous wastes into nitrate (Figures 4.33 and 4.34). The media beds themselves act as both mechanical filters and biofilters when using that technique, but additional mechanical filtration is sometimes necessary for higher fish densities (15 kg/m3). Without the media beds, such as in NFT and DWC units, standalone filtration is necessary. Mineralization of solid wastes returns more nutrients to the system. Mineralization occurs in media beds, but within NFT and DWC systems separate apparatus are needed. 4.2.3 Hydroponic components – media beds, NFT, DWC The hydroponic component is the term to describe the plant-growing sections in the unit. There are several designs, three of which are discussed in detail in this publication, but each warrants a separate section. These three designs are: media bed units, sometimes called

particulate beds, where plants grow within a substrate (Figures 4.35 and 4.36); nutrient film technique (NFT) units, where plants grow with their roots in wide pipes supplied with a trickle of culture water (Figure 4.37 and 4.38); and deep water culture (DWC)

FIGURE 4.34 Top view of mechanical solids separator (right) connected to

the biofilter (left)

FIGURE 4.33 Diagram of a mechanical solids separator (right) connected to

the biofilter (left)

Water from fish tankWater to grow beds

Biofilter media

Pump

FIGURE 4.36Different vegetable plants growing in the same

media bed

FIGURE 4.35Vegetables growing in a media bed unit


units, also called raft aquaponics or floating bed systems, where plants are suspended above a tank of water using a floating raft (Figure 4.39 and 4.40). Each method has advantages and disadvantages, all with different component styles to suit the needs of each method. See Sections 4.3–4.6 for details of each.

4.2.4 Water movementWater movement is fundamental for keeping all organisms alive in aquaponics. The flowing water moves from the fish tanks, through the mechanical separator and the biofilter and finally to the plants in their media beds, pipes or canals, removing the dissolved nutrients. If water movement stops, the most immediate effect will be a reduction in DO and accumulation of wastes in the fish tank; without the mechanical filter and biofilter fish can suffer and die within a few hours. Without water flow, the water in media beds or DWC units will stagnate and become anoxic, and NFT systems will dry out.

A commonly cited guideline for densely-stocked aquaponic systems is to cycle the water two times per hour. For example, if an aquaponic unit has a total water volume of 1 000 litres, the water flow rate should be 2 000 litres/h, so that every hour the water is cycled two times. However, at low stocking densities this turnover rate is unnecessary, and the water only needs to be cycled one time per hour. There are three commonly used methods of moving water through a system: submersible impeller pumps, airlifts and human power.

Submersible impeller water pumpMost commonly, an impeller-type submersible water pump is used as the heart of an aquaponics unit, and this type of pump is recommended (Figure 4.41). External pumps

FIGURE 4.38Lettuce growing in square pipes of a nutrient film

technique unit

FIGURE 4.37Detail of lettuce plants growing in circular pipes of a

nutrient film technique unit

FIGURE 4.40Lettuce growing densely in small deep water

culture unit

FIGURE 4.39Swiss chard (Beta sp.) suspended on a polystyrene

raft in a deep water culture canal


could be used, but they require further plumbing and are more appropriate for larger designs. High-quality water pumps should preferably be used in order to guarantee a long life span and energy efficiency. Top-quality pumps will maintain their pumping capacity and efficiency for least 1–2 years, with an overall life span of 3–5 years, whereas inferior products will lose their pumping power in a shorter time leading to significantly reduced water flows. Regarding flow rate, the small-scale units described in this publication need a flow rate of 2 000 litres/h at a head height of 1.5 meters; a submersible pump of this capacity would consume 25–50 W/h. A helpful approximation to calculate energy

efficiency for submersible pumps is that a pump can move 40 litres of water per hour for every watt per hour consumed, although some models claim twice this efficiency.

When designing the plumbing for the pump, it is important to realize that pumping power is reduced at every pipe fitting; up to 5 percent of the total flow rate can be lost at each pipe connection when water is forced through. Thus, use the minimal number of connections between the pump and the fish tanks. It is also important to note that the smaller the diameter of the pipes, the larger the water flow loss. A 30 mm pipe has twice the flow of a 20 mm pipe even if served from pumps with same capacity. In addition, a larger pipe does not require any maintenance to remove the buildup of solids accumulating inside. In practical terms, this results in significant savings on electricity and operating costs. When installing an aquaponic unit, be sure to place the submersible pump in an accessible location because periodic cleaning is necessary. Indeed, the internal filter will need cleaning every 2–3 weeks. Submersible water pumps will break if they are run without water; never run a pump dry.

AirliftAirlifts are another technique of lifting water (Figure 4.42). They use an air pump rather a water pump. Air is forced to the bottom of a pipe within the fish tank, bubbles form and burst, and during their rise to the surface the bubbles transport water with them. One benefit is that airlifts can be more electrically efficient, but only at small head heights (30–40 cm). Air lifts gain power in deeper tanks, and are best at a depth greater than one metre. An added value is that airlifts do not clog the way that submersible impeller-type pumps do. In addition, water is also oxygenated through the vertical movement operated by the air bubbles. However, the volume of air pumped should be adequate to move the water along the pipe. Air pumps generally have a longer life than submersible water pumps. The main benefit comes from an economy of scale – a single air pump can be purchased for both aeration and water circulation, which reduces the capital investment in a second pump.

FIGURE 4.41Submersible water pump, commercially available in many brands, used in small-scale aquaponic units

FIGURE 4.42Simple water airlift


Human powerSome aquaponic systems have been designed to use human power to move water (Figure 4.43). Water can be lifted in buckets or by using pulleys, modified bicycles or other means. A header tank can be filled manually and allowed to slowly drain throughout the course of the day. These methods are only applicable for small systems, and should only be considered where electricity is unavailable or unreliable. Often these systems will have low DO and insufficient mixing of nutrients, although they can be used successfully in conjunction with some modified techniques discussed in Chapter 9. 4.2.5 AerationAir pumps inject air into the water through air pipes and air stones that lie inside the water tanks, thereby increasing the DO levels in the water (Figure 4.44). Additional DO is a vital component of NFT and DWC units. Air stones are located at the end of the air line, and serve to diffuse the air into smaller bubbles (Figure 4.45). Small bubbles have more surface area, and therefore release oxygen into water better than large bubbles; this makes the aeration system more efficient and contributes to saving on costs. It is recommended that quality air stones be used in order to obtain the smallest air bubbles. Biofouling will occur, and air stones should be cleaned regularly first with a chlorine solution to kill bacterial deposits and then, if necessary, with a very mild acid to remove mineralization, or replaced, when the flow of bubbles is inconsistent. Quality air pumps are an irreplaceable component of aquaponic systems, and many systems have been saved from catastrophic collapse because of an abundance of DO. If possible, it is preferable to use a combination AC/DC air pump in case of electricity shortages, because when disconnected from AC power during an outage, the charged DC batteries can continue working.

Sizing aeration systemsFor small-scale units, with about 1 000 litre fish tanks, it is recommended that at least two air lines, also called injectors, with air stones should be placed in the fish tank, and one injector in the biofilter container. To understand the volume of air entering the system, it is worth measuring the flow rate. To do this, simply invert a volumetric measuring device (a 2 litre bottle, measuring cup, graduated beaker) in the fish tank. With the help of an assistant, begin

FIGURE 4.43Backyard aquaponic system without the use of a

water pump

FIGURE 4.44Small air pump commercially available in

many brands

FIGURE 4.45Air stone used to diffuse pressurized air into fine

bubbles in the water


a stopwatch at the same time as the bubbling air stone is inserted into the measuring device. Stop the stopwatch when the container is full of air. Then, determine the flow rate in litres per minute using a ratio. The target for systems described here is 4–8 litres/min for all of the air stones combined. It is always better to have extra DO rather than not enough.

Try to place air stones so that they do not re-suspend settling solids, thus preventing their removal through the centre drain.

Venturi siphonsLow-tech and simple to construct, Venturi siphons are another technique to increase the DO levels in aquaponics. This technique is especially valuable in DWC canals. Simply speaking, Venturi siphons use a hydrodynamic principle that pulls in air from the outside (aspiration) when pressurized water flows with a faster speed through a pipe section of a smaller diameter. With constant water flow, if the pipe diameter diminishes the water velocity must increase, and this faster speed creates a negative pressure. Venturi siphons are short sections of pipe (20 mm diameter, 5 cm length) inserted inside the main water pipe of a larger diameter (25 mm). As the water in the main pipe is forced through the narrower section, it creates a jet effect (Figure 4.46). This jet effect sucks surrounding air into the water stream through a small hole cut into the outer constriction pipe. If the Venturi siphon is underwater, the small hole can be connected to a length of tubing that is exposed to the atmosphere. Venturi siphons can be integrated into each inflow pipe in DWC canals, which will raise the DO content of the canal. They can also serve as a redundancy for fish tank aeration if the air pump fails. See the section Further Reading for more sources of information.

4.2.6 Sump tankThe sump tank is a water collection tank at the lowest point in the system; water always runs downhill to the sump (Figure 4.47). This is often the location of the submersible pump. Sump tanks should be smaller than the fish tanks, and should be able to hold

FIGURE 4.46Step by step preparation of a Venturi siphon. A small section of pipe (a) is inserted into the end of the main water pipe (b). A small notch is cut (c, d) into the narrower pipe through

which air is sucked (e)

c d e

ba

53

between one-fourth and one-third of the volume of the fish tank. For ebb-and-flow type media beds, the sump needs to be large enough to hold at least the entire volume of water in the grow beds (see Section 4.3). External sump tanks are mainly used in media bed units; however, for DWC units the actual hydroponic canal can be used as a sump tank / pump house also. Although helpful, it is not an essential system component, and many designs do not employ an external sump tank. Very small units, with fish tanks up to 200 litres can simply pump water from the fish tank to the grow beds, from where water trickles back down into the fish tank. However, for larger units it is very useful to have a sump.

A common method of aquaponics, and the one recommended here, is to have the pump located in the sump tank. A commonly used acronym describes the key points of this design, which is: constant height in fish tank – pump in sump tank (CHIFT–PIST). Using this method means that any water losses, including both evaporation and leaking components, are only manifested within the sump tank and do not affect the volume of the fish tank. It is then straight-forward to measure the normal evaporative losses and to calculate how often water needs replenishing, and it can be determined immediately if there is a leak. Perhaps more importantly, any leaks in the hydroponic system will not harm the fish. Section 9.2 discusses securing water levels in different ways.

4.2.7 Plumbing materialsEvery system requires a selection of PVC pipe, PVC connections and fittings, hoses and tubes (Figure 4.48). These provide the channels for water to flow into each component. Bulkhead valves, Uniseals® (hereafter uniseal), silicone sealant and Teflon tape are also needed. The PVC components are connected together in a permanent way using PVC cement, although silicone sealant can be temporarily used if the plumbing is not permanent and the joints are not under high water pressure. In addition, some general tools are needed such as hammers, drills, hand saws, electric saws, measuring tapes, pliers, channel-locking pliers, screwdrivers, levels, etc. One special tool is a hole-saw and/or spade bit, which are used in an electric drill to make holes up to 8 cm, necessary for inserting the pipes into the fish tanks and filters, as well as for making holes in the PVC or polystyrene grow beds in NFT and DWC systems. Appendix 8 contains a detailed list of materials needed for each unit described in this publication.

Make sure that the pipes and plumbing used in the system have never previously been used to hold toxic substances. It is also important that the plumbing used is of food-grade quality to prevent possible leeching of chemicals into the system water. It is also important to use pipes that are black and/or non-transparent to light, which will stop algae from growing.

Design of aquaponic units

FIGURE 4.48A selection of commonly used plumbing materials

FIGURE 4.47Sump tank buried in the ground to allow water

collection by gravity


4.2.8 Water testing kitsSimple water tests are a requirement for every aquaponic unit. Colour-coded freshwater test kits are readily available, fairly economical and easy to use, and thus these are recommended. These can be purchased in aquarium stores or online. These kits include tests for pH, ammonia, nitrite, nitrate, GH and KH (Figure 4.49). Be sure that the manufacturers are reliable and that the expiration date is still valid. Other methods include digital meters or test strips. If using digital meters for pH or nitrate, be sure to calibrate the units according to the manufacturer’s directions. A thermometer is necessary to measure water

temperature. In addition, if there is risk of saltwater in the source water, a cheap hydrometer, or a more accurate but more expensive refractometer, is worthwhile. More details on the use of colourimetric test kits are included in Section 3.3.6.

4.3 THE MEDIA BED TECHNIqUEMedia-filled bed units are the most popular design for small-scale aquaponics. This method is strongly recommended for most developing regions. These designs are efficient with space, have a relatively low initial cost and are suitable for beginners because of their simplicity. In media bed units, the medium is used to support the roots of the plants and also the same medium functions as a filter, both mechanical and biological. This double function is the main reason why media bed units are the simplest; the following sections demonstrate how NFT and DWC methods both require isolated and more complicated components for filtration. However, the media bed technique can become unwieldy and relatively expensive at a larger-scale. Media can become clogged if fish stocking densities exceed the beds’ carrying capacity, and this can require separate filtration. Water evaporation is higher in media beds with more surface area exposed to the sun. Some media are very heavy.

There are many designs for media beds, and this is probably the most adaptable technique. For example, Bumina is an aquaponic technique used in Indonesia that uses many small media beds connected to an in-ground fish tank (Section 9.4.3). Moreover, recycled materials can easily be repurposed to hold the media and the fish.

4.3.1 Water flow dynamicsFigure 4.50 shows the main components of an aquaponic system using media beds, including the fish tank, the media beds, the sump tank and water pump, as well as concrete blocks for support. It is easiest to understand by following the water flow through the system. Water flows by gravity from the fish tank, through a simple mechanical filter and into the media beds. These media beds are full of porous biofilter media that serves as both the mechanical and biological filter and location for mineralization. These beds both host the colony of nitrifying bacteria as well as provide the place for the plants to grow. On exiting the media beds, the water travels down to the sump tank, again by gravity. At this point, the water is relatively free of solid and dissolved wastes. Finally, this clean water is pumped back to the fish tank, which causes the water level to rise and over-flow from the fish tank back into the media beds, completing the cycle. Some media beds are designed to flood-and-drain, which means that the water level rises to a certain point and then completely drains. This adds oxygen to the plant roots and aids in the biofiltration of the ammonia. Other media irrigation methods use a constant flow of water, either entering one side of the bed and exiting the other, or distributed through a drip irrigation array.

FIGURE 4.49Water test kit, available in many brands, including

tests for ammonia, nitrite, nitrate, pH and alkalinity


4.3.2 Media bed construction MaterialsMedia beds can be made from plastic, fibreglass or a wooden frame with water-tight rubber or polyethylene sheeting on the base and inside the walls. The most popular “do-it-yourself” (DIY) media beds are made from plastic containers, modified IBCs or even old bathtubs (Figure 4.51). It is possible to use all of the above as beds and other kinds of tanks so long as they meet these following requirements:

•strongenoughtoholdwaterandgrowingmedia without breaking;

•abletowithstanddifficultweatherconditions;

•madeoffood-gradematerialthatissafeforthe fish, plants and bacteria;

•canbeeasilyconnectedtootherunitcomponentsthroughsimpleplumbingparts;and

•canbeplacedincloseproximitytotheotherunitcomponents.

Shape The standard shape for media beds is a rectangle, with a width of about 1 m and a length of 1–3 m. Larger beds can be used / manufactured, but they require additional support (i.e. concrete blocks) in order to hold their weight. In addition, longer beds may have unequal distributions of solids that tend to accumulate at the water inlet, raising the risk of anaerobic spots. The beds should not be so wide that the farmer/operator is unable to reach across, at least half-way.

DepthMedia bed depth is important because it controls the amount of root space volume in the unit which determines the types of vegetables that can be grown. If growing large fruiting vegetables such as tomatoes, okra or cabbage, the media bed should have a depth of 30 cm, without which the larger vegetables would not have sufficient root space, would experience root matting and nutrient deficiencies, and would probably topple

FIGURE 4.50Illustration of a small media bed unit


FIGURE 4.51Media bed unit constructed from intermediate

bulk containers


over (Figure 4.52). Small leafy green vegetables only require 15–20 cm of media depth, making them a good choice if the media bed size is limited. Even so, some experiments have shown that even the larger crops can be grown in shallow beds if the nutrient concentrations are sufficient.

4.3.3 Choice of medium All applicable growing media will have several common and essential criteria. The medium needs to have adequate surface area while remaining permeable for water and air, thus allowing the bacteria to grow, the water to flow and the plants roots to breathe. The medium

must be inert, not dusty, and non-toxic, and it must have a neutral pH so as not to affect the water quality. It is important to wash the medium thoroughly before placing it into the beds, particularly volcanic gravel which contains dust and tiny particles. These particles can clog the system and potentially harm the fishes’ gills. Finally, it is important to work with material that is comfortable for the farmer. These essential criteria are listed below:

• largesurfaceareaforbacterialgrowth;•neutralpHandinert(meaningthemediumwillnotleachoutanypotentiallytoxic

substances);•gooddrainageproperties;•easytoworkwith;•sufficientspaceforairandwatertoflowwithinthemedium;•availableandcost-effective;and• light-weight,ifpossible.

Several common media meeting the criteria are discussed include:5</<<<,

Volcanic gravel (tuff)Volcanic gravel is the most popular medium to use for media bed units and is recommended where available (Figure 4.53). The three best qualities of volcanic gravel are that it has a very high surface area to volume ratio, it can be cheap and easy to obtain, and it is almost chemically inert. Volcanic gravel has a surface area to volume

ratio of about 300 m2/m3, depending on the particle size, which provides ample space for bacteria to colonize. Volcanic gravel is abundant in many locations around the world. Once washed of dust and dirt, volcanic gravel is almost completely chemically inert, except for small releases of microelements such as iron and magnesium and the absorption of phosphate and potassium ions within the first few months of starting a unit. The recommended size of volcanic gravel is 8–20 mm in diameter. Smaller gravel is likely to clog with solid waste and larger gravel does not offer the surface area or plant support as required.

FIGURE 4.53Volcanic tuff used as growing medium

FIGURE 4.52Fibreglass tanks used in a media bed unit


LimestoneLimestone is not recommended as a growing medium, though it is commonly used (Figure 4.54). Limestone, a sedimentary rock, is less desirable than other media because it has a lower surface area to volume ratio, is heavy and is not inert. Limestone is composed primarily of calcium carbonate (CaCO3), which dissolves in water and affects water quality. Limestone will increase the KH of the water, which will also increase the pH (see Section 3.3). Therefore, this material is better used where water sources are very low in alkalinity or acidic, as in cases of alkaline water it would call for constant acid corrections of incoming waters. Nevertheless, a small addition of limestone can help to counterbalance the acidifying effect of nitrifying bacteria, which can offset the need for regular water buffering in well balanced systems. Limestone may not be as comfortable to work with in terms of planting and harvesting, and it can experience clogging if the proper granulometry is not chosen. However, it is often the cheapest and most common form of gravel available. Limestone is only acceptable as a medium if no other media are available, but be aware of its impact on water quality.

Light expanded clay aggregateLight expanded clay aggregate (LECA) consists of expanded clay pebbles (Figure 4.55). Originally, it was manufactured for thermal insulation of building roofs, but it has more recently been used in hydroponics. These pebbles are round in shape and very lightweight compared with other substrates. They are very comfortable to work with and ideal for rooftop production. The surface area of LECA is about 250–300 m2/m3, which is within the target range. However, LECA is relatively expensive and not widely available around the world. It comes in a variety of sizes; for aquaponics the larger sizes with diameters 8–20 mm are recommended. This material can give additional benefits to growers in case of media beds placed directly on rooftop floors (depending on design). The building can in fact benefit from additional insulation, which can decrease houses’ cooling/heating costs.

Other possible media choicesIf the above-listed media are unavailable, it is possible to use other media. Alternatives include: river-bed gravel, which is usually limestone but can have a low surface area to volume ratio depending on the granulometry; pumice (also rockwool), a white/grey volcanic material also popularly used as growing medium in hydroponics; recycled plastic, although plastic floats and needs to be held submerged with a layer of gravel on top; or organic substrates such as coconut fibre, sawdust, peat moss or rice hull, which are often inexpensive but risk becoming anoxic, deteriorating over time and clogging the system. However, organic substrate can be used for a time within aquaponics, and once it begins to deteriorate, the media can be removed from the system, composted, and used as a valuable soil addition for soil crops. Table 4.1 summarizes the major characteristics for all the growing media mentioned above.

FIGURE 4.54Limestone gravel used as growing medium

FIGURE 4.55Light expanded clay aggregate pellets used as

growing medium


Displacement of water by mediaDepending on the medium, it will occupy roughly 30–60 percent of the total media bed volume. This percentage will help decide on the sump tank size for each unit, because the sump tank, at the very least, will need to hold the total water volume contained in all the media beds. Sump tanks should be slightly oversized to ensure that there is always adequate water for the pump to run without ever running dry.

For example, for a media bed of 1 000 litres (dimensions 2 m long × 2 m wide × 0.25 m medium depth), the growing medium will displace 300–600 litres of this space, and therefore the water volume of the media bed would be 400–700 litres. It is recommended that the sump volume be at least 70 percent of the total media bed volume. For this example, the sump tank should be approximately 700 litres.

4.3.4 FiltrationThe media beds serve as very efficient filters, both mechanical and biological. Unlike the NFT and DWC systems (discussed below), the media bed technique utilizes a combination filter and plant growing area. In addition, the media bed provides a place for mineralization to occur, which is absent in the NFT and DWC systems. However, at high stocking densities (>15 kg/m3), the mechanical filtration can be overwhelmed and can face the risk of having the media clogged and producing dangerous anaerobic spots.

Mechanical filter The medium-filled bed functions as a large physical filter, capturing and containing the solid and suspended fish waste and other floating organic debris. The effectiveness of this filter will depend on the particle size of the medium because smaller particles are more densely packed and capture more solids. Moreover, a high water flow rate can force particles through the media bed and escape the filter. Over time, the captured solid wastes will break down and be mineralized. A properly balanced system will process all of the incoming solid wastes.

When media beds are improperly sized for the stocking density, the media bed can become clogged with solids. This indicates a mistake in the original design when the feed rate ratio was used to balance the system. This situation leads to beds clogged with solid waste, poor water circulation, anoxic areas and dangerous conditions. When this occurs, the medium needs to be washed, which is labour-intensive, disrupts the plant growing cycle and can briefly disturb the nitrifying bacteria.

To avoid this situation be sure that the original design considered the stocking density, feeding regime, and used the feed rate ratio to calculate the required area of the media bed. Alternatively, another solids capture device can be integrated into the

TABLE 4.1 Characteristics of different growing media

Media type Surface

area (m2/m3)

pH Cost Weight Lifespan Water retention

Plant support

Ease to work with

Volcanic gravel (tuff) 300–400 Neutral Medium Medium Long Medium–

Poor Excellent Medium

Volcanic gravel (pumice) 200–300 Neutral Medium–

High Light Long Medium Medium–Poor Easy

Limestone gravel 150–200 Basic Low Heavy Long Poor Excellent Difficult

Expanded clay(LECA) 250–300 Neutral High Light Long Medium–

Poor Medium Easy

Plastic bottle caps 50–100 Inert Low Light Long Poor Poor Easy

Coconut fibre 200–400(variable) Neutral Low–

Medium Light Short High Medium Easy


unit design. This is also recommended where the stocking density exceeds 15 kg/m3 and/or if the feeding rate is above 50 g/day for each square metre of grow bed. There are several options for this additional mechanical filter. A rudimentary and cheap technique is to affix an old, orphan sock to the tap where water from the fish tank enters the media bed. This simple filter can be removed each day and rinsed. Another more elaborate method is to place a 3–5 litre bucket inside the media bed with several small holes (6–8 mm) drilled into the side surfaces (Figure 4.31). Sponges, nylon netting or even growing media (volcanic gravel, LECA) can be tied in a porous inert net bag and placed into this bucket. This filter will trap the solid waste, and the filter can then be removed periodically to be rinsed and replaced.

Biological filtrationAll of the growing media herein outlined have a large surface area where nitrifying bacteria can colonize. Of all of the aquaponic designs, media beds have the most biological filtration because of the huge area of media on which the bacteria can grow. Biofiltration capacity can be limited or lost if the media beds become anoxic, if the temperatures drop or if the water quality is poor, but generally media beds have more than adequate biological filtration.

MineralizationOver time, the solid and suspended fish waste and all other debris are slowly broken down by biological and physical processes into simple nutrients in the form of simple molecules and ions that the plants can easily absorb. If sludge accumulates in the media bed and does not leave, it may indicate that the mineralization process is not sufficient. In this case, the recommendation is to use more effective mechanical filtration and process the filtered waste separately. This process is described in more detail in Section 4.2.2 and Chapter 5.

4.3.5 The three zones of media beds – characteristics and processesThe nature of a flood-and-drain media bed creates three separate zones that can be considered microecosystems, which are differentiated by their water and oxygen content. Each zone hosts a diverse group of bacteria, fungi, micro-organisms, worms, insects and crustaceans. One of the most important is the nitrifying bacteria used for biofiltration, but there are many other species that all have a role in the breaking down of fish wastes. It is not essential to be aware of all these organisms, but this section briefly outlines the differences between these three zones and some of the ecological processes occurring in each.

Dry zoneThe top 2–5 cm of the bed is the dry zone (Figure 4.56). This zone functions as a light barrier, preventing the light from hitting the water directly which can lead to algal growth. It also prevents the growth of fungus and harmful bacteria at the base of the plant stem, which can cause collar rot and other plant diseases. Another reason to have a dry zone is to minimize evaporation from beds by covering the wet zone from direct light. Moreover, beneficial bacteria are sensitive to direct sunlight.

Dry/wet zoneThis is the zone that has both moisture and high gas exchange. In flood-and-drain techniques (discussed below) this is the 10–20 cm space where the media bed intermittingly floods and drains (Figure 4.57). If not using flood-and-drain techniques, this zone will be the path that the water flows through the medium. Most of the biological activity will occur in this zone. The root development, the beneficial bacteria colonies and beneficial micro-organisms are active in this zone. The plants and the


animals receive their water, nutrients and oxygen because of the interface between air and water.

One common technique is adding worms to the media bed which will live in this dry/wet zone. The worms will contribute to the breakdown of solid fish waste and they will also consume any dead leaves or roots. This activity will prevent wastes from clogging the system. See Section 9.1.1 for more information about worms and vermicompost.

Wet zoneThis zone, the bottom 3–5 cm of the bed, remains permanently wet. In this zone, the small particulate solid wastes accumulate, and, therefore the organisms that are most active in mineralization are located here. These include heterotrophic bacteria and other micro-organisms. These organisms are responsible for breaking down the waste into smaller fractions and molecules that can be absorbed by the plants through the process of mineralization.

Dry zone

Wet & dry zone

Wet zone

Water outflow

FIGURE 4.56The three zones of a media bed during the drain cycle

Dry zone

Wet & dry zone

Wet zone

Water outflow

FIGURE 4.57The three zones of a media bed during the flood cycle


4.3.6 Irrigating media bedsThere are different techniques to deliver water to media beds, each can be relevant depending on the local availability of materials, the degree of technology desired or the experience of the operators. Water can be simply trickled from holed pipes uniformly distributed on the medium; this is a perfectly acceptable design. Some experts have demonstrated that constant flow designs, where the water level within the grow bed is always the same, support the same growth rates of plants as more complicated methods. These water distribution systems can become clogged with solid fish waste and should be periodically cleared.

A method called flood-and-drain, also known as ebb-and-flow, can be used where the system of plumbing causes the media beds to flood with water from the fish tank and then drain back in the sump tank. This is accomplished through autosiphons or timed pumping. This alternation between flooding and draining ensures that the plants have both fresh nutrients and adequate air flow in the root zone. This thereby replenishes the oxygen levels for plants and bacteria. It also ensures that enough moisture is in the bed at all times so the bacteria can thrive in their optimum conditions. Usually, these systems go through the complete cycle 1–2 times every hour, but some successful systems only cycle 3–4 times per day. Flood-and-drain designs are not the only techniques for media beds, and managing the water flow cycle may be frustrating and time-consuming for novice operators.

This publication briefly discusses two popular methods for flooding and draining a bed, although other methods, such as the looped siphon, exist and are the subject of current research.

Bell siphonThe bell siphon is a type of autosiphon that exploits a few physical laws of hydrodynamics and allows the media bed to flood and drain automatically, periodically, without a timer (Figure 4.58). The action, timing and ultimate success of the siphon are dependent on the water’s flow rate into the bed, which is constant. Bell siphons can nevertheless be finicky and require attention.

Water flow dynamicsWater flows into each grow bed at a constant flow rate. As the water fills the grow bed it reaches the top of the standpipe, and begins to drip through the standpipe back to the sump tank. Without the bell portion of the bell siphon, this would create a condition

BellStand pipe

Media guard Grow bed

Water outflow

FIGURE 4.58Diagram of a bell siphon and components installed in a grow bed


of constant water height. Instead, as the water continues to fall through the standpipe, the bell, which sits over the standpipe something like a hat, acts as an air tight lock and produces a siphon effect. This suction within the bell starts the siphon. Once started, all the water from the bed starts to rapidly flush down the standpipe as the bell keeps its air tight seal. The draining through the standpipe is faster than the constant inflow from the fish tank. When the water in the grow bed drains all the way down to bottom, air enters the bottom of the bell and immediately stops the siphon. The water then slowly fills back up and repeats the whole cycle again continuously. See the section on Further Reading at the end of this publication for more information on bell siphons.

Main components of a bell siphon The three main components of a bell siphon are described below. Note that detailed instructions for understanding, constructing and optimizing bell siphons, as well as pictures of these components, can be found in Appendix 8. The dimensions of the standpipe, bell and media guard are completely dependent on the size of the grow bed and incoming water flow rate. These dimensions are provided for the aquaponic designs outlined in this publication for a media bed of 1–3 m2 with a media depth of 30 cm, with an incoming water flow rate of 200–500 litres/h for each bed. For large grow beds, all of the components would be larger.

Standpipe – The standpipe is constructed of a PVC pipe, 2.5 cm diameter, of a height of 22 cm. The standpipe passes through the bottom of the grow bed, connecting to the sump, and is the path of the water as it drains.Bell – The bell is a PVC pipe, 7.5 cm diameter, of a height of 25 cm. The pipe is capped with a PVC end-cap on top, and is open on the bottom where it fits over the standpipe. Two rectangular gaps, 1 cm × 4 cm, are located near the bottom of the bell, 1.5 cm up on opposite sides, through which the water is pulled up into the standpipe inside the bell. A final 1 cm hole is drilled 5 cm from the bottom to help break the siphon once the grow bed is drained by allowing air to enter.Media guard – The media guard is a PVC pipe, 11 cm diameter, of a height of 32 cm with many small holes drilled in its sides. The media guard prevents the gravel from the grow bed from entering and clogging the standpipe, without obstructing the flow of water.

Timer mechanismThis method of flood-and-drain irrigation relies on a timer switch on the water pump to control the periodic flooding and draining (Figure 4.59). The benefit of this method

Water outflow

FIGURE 4.59Diagram of a media bed standpipe and media guard


is that there is no autosiphon, which can be labour-intensive to calibrate. However, the reduced water circulation and reduced aeration in the fish tanks results in less overall filtration. This method is less appropriate in high-density stocking situations, and requires careful attention to provide supplemental aeration to fish.

Water flow dynamicsWater flows into the grow bed, flooding the bed until the water reaches the top of the standpipe. The water then drains through this standpipe and down into the sump tank. The large standpipe is of sufficient diameter to drain all of the inflowing water; the top of the large standpipe is the deepest flood that the grow bed will experience. There is also a small inlet, 6–12 mm diameter, into this same standpipe located near the bottom. This small inlet is insufficient to drain all of the incoming water and, therefore, even as water enters the small inlet, the grow bed continues to flood until it reaches the top. At some point after the bed is full, the timer cuts the power to the water pump. Water in the media bed begins to flow out through the small inlet hole, continuing to drain the grow bed until the water reaches the level of the bottom hole. At this point, the power is returned to the water pump and the grow bed is refilled with fresh fish-tank water. It is very important that the water flowing into the media bed is greater than the water flowing through the small inlet in the standpipe so that the bed will fully flood again. The flooding and draining cycle length and the diameter of the dripping hole are determined by the size of the media bed and the incoming flow rate.

To ensure adequate filtration, the entire fish tank volume should be pumped through the grow beds every hour. Finally, make sure to flush the beds out once every week by temporarily removing the standpipe and allowing the remaining water to drain.

The materials involved for the timer method for the aquaponic designs included in this publication are as follows: a standpipe, 2.5 cm diameter, of a height of 23 cm that has a secondary dripping hole, 6–12 mm diameter, 2.5 cm above the bottom; a media guard, 11 cm diameter and 32 cm in height, encircling the standpipe to prevent media from clogging it; and a timer that controls the pump that is calibrated based on the flow rate of the pump and the drain rate of the standpipe.

4.4 NUTRIENT FILM TECHNIqUE (NFT)The NFT is a hydroponic method using horizontal pipes each with a shallow stream of nutrient-rich aquaponic water flowing through it (Figure 4.60). Plants are placed within holes in the top of the pipes, and are able to use this thin film of nutrient-rich water.

FIGURE 4.60Illustration of a small nutrient film technique unit



Both the NFT and DWC are popular methods for commercial operations as both are financially more viable than media bed units when scaled up (Figure 4.61). This technique has very low evaporation because the water is completely shielded from the sun. This technique is far more complicated and expensive than media beds, and may not be appropriate in locations with inadequate access to suppliers. This technique is most useful in urban applications, especially when using vertical space or weight-limitations are considerations.

Although all methods have a different approach to actually growing plants, the most important difference between them is the method

of filtration that both the NFT and DWC units utilize compared with the media bed method. The following text describes this method of filtration for NFT and DWC units in detail. Afterwards, the NFT and DWC methods are discussed individually. The general layout of this section begins with water flow dynamics, or how the water moves through the system. Then filtration methods are discussed, followed by specific planting guidelines for NFT systems.

4.4.1 Water flow dynamicsThe water flows by gravity from the fish tank, through the mechanical filter and into the combination biofilter/sump. From the sump, the water is pumped in two directions through a “Y” connector and valves. Some water is pumped directly back to the fish tank. The remaining water is pumped into a manifold that distributes the water equally through the NFT pipes. The water flows, again by gravity, down through the grow pipes where the plants are located. On exiting the grow pipes, the water is returned to the biofilter/sump, where again it is pumped either into the fish tank or grow pipes. The water that enters the fish tank causes the fish tank to overflow through the exit pipe and back into mechanical filter, thus completing the cycle.

This design, as described in this publication, is called a “Figure 8” design because of the path of the water. This design ensures that filtered water enters both the fish tank and the grow pipes, while only using one pump. There is no need to place the sump lower than the rest of the unit, making this design possible to use on existing concrete floors or on rooftops. All components are at a comfortable working level for the farmer without stooping or using ladders. Moreover, the design fully utilizes the size of the IBC container to ensure adequate room for the fish. One drawback is that the combination sump/biofilter works to dilute the nutrient concentration of the water reaching the grow pipes, and at the same time, returns water to the fish before the water has been fully stripped of nutrients. However, the slight dilution is managed by controlling the bidirectional flow leaving the sump/biofilter and, overall, it has little effect on the efficacy of this system in light of the benefits provided. Generally, the pump returns 80 percent of the water to the fish tanks and the remaining 20 percent to the grow beds or canals, and this can be controlled with the valve.

4.4.2 Mechanical and biological filtrationDedicated filtration is of critical importance in both NFT and DWC units. Whereas the medium in the media bed technique serves as a biofilter and a mechanical filter, the NFT and DWC techniques do not have this luxury. Therefore, both types of filters need to be deliberately constructed: first, a physical trap to catch the solid wastes, and then a biological filter for nitrification. As mentioned in Section 4.3, there are many

FIGURE 4.61Lettuce growing in a commercial nutrient film

technique unit


types of mechanical filters, and NFT and DWC units require those at the high end of the spectrum outlined therein. The designs described in Appendix 8 use a mechanical swirl filter to trap particulate wastes, with periodic venting of the captured solids. On exiting the swirl filter, the water passes through an additional mesh screen to trap any remaining solids and then reaches the biofilter. The biofilter is well oxygenated with air stones and contains a biofiltration media, usually Bioballs®, nylon netting or bottle caps, where the nitrifying bacteria transform the dissolved wastes. With insufficient filtration, both NFT and DWC units would clog, become anoxic and exhibit poor growing conditions for plants and fish alike.

4.4.3 Nutrient film technique grow pipes, construction and plantingFollowing on from the filtration methods explained above, NFT then employs the use of plastic pipes laid out horizontally to grow vegetables using the aquaponic water (Figure 4.62). Where possible, use pipes of rectangular section with width larger than height, which is standard among hydroponic growers. The reason lies in a larger film of water that hits the roots with the scope of increasing the nutrient uptake and plant growth. One of the benefits of the NFT is that the pipes can be arranged in many patterns, beyond the scope of this publication, and can make use of vertical space, walls and fences, and overhanging balconies (Figure 4.63).

The water is pumped from the biofilter into each hydroponic pipe with a small equal flow creating a shallow stream of nutrient-rich aquaponic water flowing along the bottom. The grow pipes contain a number of holes along the top of the pipe into which the plants are placed. As the plants start to consume the nutrient-rich water from the stream, they begin to develop root systems inside the grow pipes. At the same time, their stems and leaves grow out and around the pipes. The shallow film of water at the bottom of each pipe ensures that the roots receive large amounts of oxygen at the root zone along with moisture and nutrition. Keeping a shallow stream allows the roots to have a larger air exchange surface. The water flow for each grow pipe should be no greater than 1–2 litres/min. The flow rate is controlled from the Y-valve, with all excess water flow returned to the fish tank.

Grow pipe shape and sizeIt is wise to choose a pipe with the optimum diameter for the types of plants grown. Pipes with a square cross-section are best, but round pipes are more common and totally acceptable. For larger fruiting vegetables, 11 cm diameter grow pipes are needed while fast-growing leafy green and small vegetables with small root masses only require pipes with a diameter of 7.5 cm. For small-scale polyculture (growing many types of vegetables) 11 cm diameter pipes should be used (Figure 4.64). This avoids plant selection limitations because the small plants can always be grown in the larger pipes, although there would be a sacrifice in planting density. Plants with extensive root

FIGURE 4.62Lettuce growing in square grow pipes of a nutrient

film technique unit

FIGURE 4.63Grow pipes of a nutrient film technique unit

arranged vertically


systems, including mature older plants, can clog smaller pipes and cause overflows and losses of water. Be especially mindful of tomatoes and mint, as their massive root systems can easily clog even large pipes.

The grow pipe length can be anywhere between 1 and 12 m. In pipes longer than 12 m, nutrient deficiencies can occur in plants towards the end of the pipes because the first plants have already stripped the nutrients. A slope of about 1 cm/m of pipe length is needed to make sure the water flows through the whole pipe with ease. The slope is controlled by using shims (wedges) on the side away from the fish tank.

PVC pipes are recommended because they are usually the most commonly available and are inexpensive. White pipes should be used as the colour reflects the sun’s rays, thereby keeping the inside of the pipes cool. Alternatively, square or rectangular hydroponic pipes with dimensions 10 cm width × 7 cm height are recommended.

Professional hydroponic pipes for commercial growers are typically this shape, and some growers use vinyl fence posts.

Planting within the grow pipesThe holes drilled into the hydroponic pipe should be 7–9 cm in diameter, and should match the size of the available net cups. There should be a minimum of 21 cm between the centre of each plant hole to allow adequate plant space for leafy greens and larger vegetables (Figures 4.65 and 4.66).

Each seedling is placed into a plastic net cup, which is then in turn placed within the grow pipe. This provides physical support for the plant. The net cups are filled with general purpose hydroponic media (volcanic gravel, rockwool or LECA) around the seedling. If desired, a 5–10 cm length of 5 cm PVC pipe can be placed inside the net cup as further balance and support to the plant. Detailed planting instructions are included in Appendix 8.

If plastic net cups are not available or are too costly, it is possible to use regular plastic drinking cups. Follow the planting procedure as outlined in the previous paragraph making sure to add many holes to the plastic drink cup so the roots have

plenty of access into the grow pipe. Other growers have had success with flexible, open-cell foam to support the plants within the grow pipe. If none of these options is available or desired, it is possible to transplant the seedlings directly into the pipes, particularly rectangular pipes (Figure 4.67). Seedlings can be transplanted with their germination medium, which will wash away into the system or the roots can be carefully rinsed, which keeps medium out of the system but can increase the transplant stress. Nevertheless, it is preferable to use net cups filled with media.

When initially planting the seedlings into the pipe, make sure the roots can touch the stream

FIGURE 4.64Several grow pipes showing hole spacing

FIGURE 4.65Plant support materials showing grow medium

and net cup


of water at the bottom of the pipe. This will ensure that the young seedlings do not become dehydrated. Alternatively, wicks can be added that trail into the water stream. In addition, it is advisable to water the seedlings with aquaponic water one week prior to transplanting them to the unit. This will help mitigate against transplant shock for the plants as they become accustomed to the new water.

4.5 DEEP WATER CULTURE TECHNIqUEThe DWC method involves suspending plants in polystyrene sheets, with their roots hanging down into the water (Figures 4.68 and 4.69). This method is the most common for large commercial aquaponics growing one specific crop (typically lettuce, salad leaves or basil, Figure 4.70), and is more suitable for mechanization. On a small-scale, this technique is more complicated than media beds, and may not be suitable for some locations, especially where access to materials is limited.

FIGURE 4.66Full size lettuce harvested from a nutrient film technique unit. Net cup and PVC extender are

clearly visible

FIGURE 4.67Lettuce plant grown without a net cup directly in

a grow pipe

FIGURE 4.68Illustration of a small deep water culture unit using a media bed as filtration



4.5.1 Water flow dynamicsThe water flow dynamics in DWC are almost identical to those through an NFT. The water flows by gravity from the fish tank, through the mechanical filter, and into the combination biofilter/sump. From the sump, the water is pumped in two directions through a “Y” connector and valves. Some water is pumped directly back to the fish tank. The remaining water is pumped into the manifold, which distributes the water equivalently through the canals. The water flows, again by gravity, through the grow canals where the plants are located and exits on the far side. On exiting the canals the

water is returned to the biofilter/sump, where again it is pumped either into the fish tank or canals. The water that enters the fish tank causes the fish tank to overflow through the exit pipe and back into mechanical filter, thus completing the cycle.

This “Figure 8” configuration describes the path of the water seen in the DWC system. As in the NFT, the water flows through the mechanical filter and the biofilter before being pumped back to the fish tank and the plant canals. One drawback in this configuration is that the combination sump/biofilter returns part of the effluent water from the plant canals back to the plants. However, unlike in the NFT where the nutrients in the small film of water flowing at root level quickly become depleted, the large volume of water contained in the DWC canals allows for considerable amounts of nutrients to be used by plants. Such nutrient availability would also suggest different system designs. A serial distribution of water along the DWC canals can be constructed by simply using a “cascade” configuration with only a single inlet serving the farthest tank. In this case, the outlet of one tank would be the inlet of the successive one, and the increased water flow would help the roots to access a higher flow of nutrients.

In the DWC system shown in Figure 4.68, water is pumped from the biofilter container into canals that have polystyrene sheets floating on top supporting the plant. The flow rate of the water entering each canal is relatively low. Generally, every canal has 1–4 hours of retention time. Retention time is a similar concept to turnover rate, and refers to the amount of time it takes to replace all the water in a container. For example, if the water volume of one canal is 600 litres and the flow rate of water entering the container is 300 litres/h, the retention time would be 2 hours (600 litres ÷ 300 litres/h).

FIGURE 4.69Illustration of a small deep water culture unit using standalone filtration

Fish tank Filter Plant growing area

FIGURE 4.70A large deep water culture unit


4.5.2 Mechanical and biological filtrationMechanical and biological filtration in DWC units is the same as in NFT units which is described in Section 4.4.2.

4.5.3 DWC grow canals, construction and plantingCanals can be of variable lengths, from one to tens of metres (Figure 4.71). In general, their length is not an issue, as seen in the NFT, because the large volume of water enables adequate nutrient supply. Optimal plant nutrition in very long canals should always allow for adequate water inflow and re-oxygenation to ensure that nutrients are not depleted and that roots can breathe. As far as the width is concerned, it is generally recommended to be the standard width of a sheet of polystyrene, but it can be multiples of this. However, narrower and longer canals enable a higher water speed that can beneficially hit the roots with larger flows of nutrients. The choice of width should also consider accessibility by the operator. The recommended depth is 30 cm to allow for adequate plant root space. Similar to fish tanks, canals can be made out of any strong inert material that can hold water. For small-scale units, popular materials include fabricated IBC plastic containers or fibreglass. Much larger canals can be constructed using wood lengths or concrete blocks lined with food-grade waterproof sheeting. If using concrete, make sure it is sealed with non-toxic, waterproof sealer to avoid potential toxic minerals leaching from the concrete into the system water.

As mentioned above, the retention time for each canal in a unit is 1–4 hours, regardless of the actual canal size. This allows for adequate replenishment of nutrients in each canal, although the volume of water and the amount of nutrients in the deep canals is sufficient to nourish the plants over longer periods. Plant growth will definitely benefit from faster flow rates and turbulent water because roots will be hit by many more ions; whereas slower flows and almost stagnant water would have a negative impact on plant growth.

Aeration for DWC units is vital. In a densely planted canal, the oxygen demand for plants can cause DO levels to plummet below the minimum. Any decomposing solid waste present in the canal would exacerbate this problem, further diminishing DO. Thus, aeration is required. The simplest method is to place several small air stones in the canals (Figure 4.72). The air stones should release about 4 liters of air per minute, and be arranged every 2–4 m2 of canal area. In addition, Venturi siphons (see Section 4.2.5) can be added to the water inflow pipes to aerate the water as it enters the canal. Finally, the Kratky method of DWC can be used (Figure 4.73). In this method, a space of 3–4 cm is left between the polystyrene and the water body inside the canal. This allows air to circulate around the top section of the plant roots. This approach removes the need for air stones in the canal as sufficient amounts of oxygen in the air are supplied to the roots. Another advantage of this method is the avoidance of direct contact of the plant

FIGURE 4.71A small-scale deep water culture aquaponic unit.

Plant roots visible below the polystyrene raft

FIGURE 4.72Air stone used inside a deep water culture canal


stems with water, which reduces the risks of plant diseases at the collar zone. Moreover, the increased ventilation as a result of the increased air space favours heat dissipation from water, which is ideal in hot climates

Do not add any fish into the canals that could eat the plant roots, e.g. herbivorous fish such as tilapia and carp. However, some small carnivorous fish species, such as guppies, mollies, or mosquito fish, can be used successfully to manage mosquito larvae, which can become a huge nuisance to workers and neighbours in some areas.

The polystyrene sheets should have a certain number of holes drilled to fit the net cups (or sponge cubes) used for supporting each plant (Figure 4.74). The amount and location of the holes is dictated by the vegetable type and the distance desired between the plants, where smaller plants can be spaced more closely. Appendix 8 includes specific details and helpful hints on how to drill the holes.

Seedlings can be started in a dedicated plant nursery (see Section 8.3) in soil blocks or a soil-less medium. Once these seedlings are large enough to handle, they can be transferred into net cups and planted into the DWC unit (Figure 4.75). The remaining space in the net cup should be filled with hydroponic media, such as volcanic gravel, rockwool or LECA, to support the seedling. It is also possible to simply plant a seed straight into the net cups on top of the media. This method is sometimes recommended if vegetable seeds are accessible because it avoids the transplant shock during replanting. When

harvesting, be sure to remove the whole plant, including roots and dead leaves, from the canal. After harvest the rafts should be cleaned but not left to dry so as to avoid killing the nitrifying bacteria on the submerged surface of the raft. Large scale units should clean the rafts with water to remove dirt and plant residues and immediately repositioned in the canals to avoid any stress to the nitrifying bacteria.

Air or oxygen roots

Water and nutrient roots

Raft

Moist air space

FIGURE 4.73An illustration of the Kratky method for deep

water culture showing the separation between the raft and the water surface

FIGURE 4.74Polystyrene sheet in a small deep water culture

unit showing planting holes

FIGURE 4.75Step by step procedure of placing a seedling and gravel (a) into a net cup (b) and placing it into the

polystyrene raft in the deep water culture unit (c)

cba


4.5.4 Special case DWC: low fish density, no filtersAquaponic DWC units can be designed that do not require external additional filtration (Figure 4.76). These units carry a very low stocking density of fish (i.e. 1–1.5 kg of fish per m3 of fish tank), and then rely mainly on the plant root space and interior area of the canals as the surface area to house the nitrifying bacteria. Simple mesh screens capture the large solid waste, and the canals serve as settling tanks for fine waste. The advantage to this method is the reduction in initial economic investment and capital costs, while at the same time eliminating the need for additional filter containers and materials, which can be difficult and expensive to source in some locations. However, lower stocking densities will lead to lower fish production. At the same time, many aquaponic ventures make the vast majority of their profits on the plant yield rather than the fish production, essentially only using the fish as a source of nutrients. Often, this method requires nutrient supplementation to ensure plant growth. If considering this method, it is worth to assessing the desired fish and plant production and considering the relative costs and gains.

Water flow dynamicsThe main difference between the two designs (high fish stocking vs. low fish stocking) is that the low- density design does not use either of the external filtration containers, mechanical or biological. Water flows by gravity from the fish tank straight into DWC canals, passing through a very simple mesh screen. Water is then returned either to a sump and pumped back to the fish tanks, or directly to the fish tanks without a sump. Water in both the fish tanks and canals is aerated using an air pump. The fish waste is broken down by nitrifying and mineralizing bacteria living on the plant root surface and the canal walls.

The fish stocking density is a continuum, stretching from very low densities that do not need filters all the way up to very high densities that need dedicated external filters. One simple solution to procure additional mineralization and biofiltration and to avoid waste accumulation of solids at the bottom of the canals consists in combining the simple mesh screen with a basket of pea gravel or clay balls positioned just above the water level where the water exits the fish tank. The basket would act as a trickling filter with its media trapping and mineralizing the solids. The water falling from the basket would also add oxygen through its splash effect. In addition, the use of pea gravel would have a buffering action against water acidification following

FIGURE 4.76Illustration of a small deep water culture without a mechanical solid separator or biofilter

Fish tank Filter Plant growing areaFish tank

Plant growing area


nitrification. Another option can include an internal biofilter within the fish tank, consisting of a simple mesh bag of biofilter material near an air stone. This can help to ensure adequate biofiltration without adding to the cost of external biofilters. Finally, increasing the overall water volume without increasing the fish stocking density, basically using large fish tanks for few fish, can help to mitigate water quality issues by diluting wastes and ensuring adequate time for the farmer to respond to changes before the fish become stressed, though this can dilute the available nutrients and hinder vegetable growth.

The lower fish density also means that the water flow rate can be lower. A smaller pump can be used, reducing the cost, but ensure that at least half of the total fish tank volume is exchanged per hour. In fact, some researchers have had success with removing the electric pump all together and relying on manual labour to cycle the water twice per day. However, these systems are utterly dependent on adequate aeration. Other than these differences, the recommendations for fish tanks and DWC canal construction are applicable for this low stocking density method.

Low stocking density unit managementThe major difference from the management of the other units, discussed in more detail in Chapter 8, is the lower stocking density. The suggested stocking density for these types of systems is 1–5 kg/m3 (compare to 10–20 kg/m3 for other systems in this manual). Previously, it has been suggested that the balance between fish and plants follows the feed rate ratio, which helps to calculate the amount of fish feed entering the system given a set growing area for the plants. These low stocking density units still follow the suggested daily feed rate ratio of 40–50 g/m2, but should be towards the lower end. A useful technique is to allow fish to feed for 30 minutes, 2–3 times per day, and then remove all uneaten food. Overfeeding will result in an accumulation of waste in the fish tanks and canals, leading to anoxic zones, poor growing conditions, diseases, and fish and plant stress. Always, but especially when using this method without filters, be sure to monitor water quality conditions closely, and reduce feeding if high ammonia or nitrite levels are detected.

Advantages and disadvantages of low stocking densityThe major advantage is a simpler unit. This system is easier to construct and cheaper to begin, having lower capital costs. The fish are less stressed because they are grown in more spacious conditions. Overall, this technique can be very useful for initial projects with low capital. These systems can be very useful for growing high-value fish, such as ornamental fish, or specialty crops, such as medicinal herbs, where the lower production is compensated with higher value.

However, a serious disadvantage is that these units are hard to scale up. Fewer plants and fish are grown in a given area, so they are less intensive than some of the systems previously outlined. In order to produce a large amount of food, these systems would become prohibitively large. Essentially, the external mechanical and biofilters are what allow aquaponics to be very intensive within a small area.

Furthermore, fish production cannot function independently from the hydroponic component; plants must be in the canals at all times. The plant roots provide the area for bacteria growth, and without these roots the biofiltration would not be sufficient to keep the water clean for the fish. If it were ever necessary to harvest all the plants at once, which can occur during disease outbreaks, season changes or major climate events, the reduced biofiltration would cause high ammonia and fish stress. On the other hand, with external mechanical and biofilters the fish production can continue without the hydroponics as a standard RAS.


4.6 COMPARING AqUAPONIC TECHNIqUESTable 4.2 below provides a quick reference and comparative summary of the various aquaponic culture systems described above.

TABLE 4.2Strengths and weaknesses of main aquaponic techniques

System type Strengths Weaknesses

Media bed units Simple and forgiving design

Ideal for beginners

Alternative/recycled parts can be used

Tall fruiting vegetables are supported

All types of plants can be grown

Multiple irrigation techniques

Many types of media can be used

High aeration when using bell siphons

Relatively low electrical energy

Medium captures and mineralizes solids

Very heavy, depending on choice of media

Media can be expensive

Media can be unavailable

Unwieldy at large scale

Higher evaporation than NFT and DWC

Labour-intensive to construct

Flood-and-drain cycles require careful calculation of water volume

Media can clog at high stocking density

Plant transplanting is more labour-intensive as the media needs to be moved

If water delivery is not uniform, plant performance may differ from bed to bed

NFT units More cost-effective than media beds on large scale

Ideal for herbs and leafy green vegetables

Minimal water loss by evaporation

Light weight system

Best method for rooftops

Very simple harvesting methods

Pipes spacing can be adjusted to suit different plants

Well researched by commercial hydroponic ventures

Smallest water volume required

Minimal labour to plant and harvest

More complex filtration method

Water pump and air pump are mandatory

Cannot directly seed

Low water volume magnifies water quality issues

Increases variability in water temperature with stress on fish

Water inlet pipes can easily clog

Vulnerable to power outages

DWC units More cost-effective method than media beds on large scale

Large water volume dampens changes in water quality

Can withstand short interruptions in electricity

Minimal water loss by evaporation

Well researched by commercial hydroponic ventures

Polystyrene rafts insulate water from heat losses/gains keeping constant temperatures

Shifting rafts can facilitate planting and harvest

Rafts provide biofilter surface area

DWC canals can be fixed with plastic liners using almost any kind of wall (wood, steel frames, metal profiles)

Can be used at multiple stocking densities

More complex filtration method

Very heavy unit

High dissolved oxygen required in the canal, and a more sophisticated air pump is required

Plastic liners must be food-grade

Polystyrene sheets are easily broken

Tall plants are more difficult to support

Large water volume increases humidity and the risk of fungal disease

4.7 CHAPTER SUMMARy•Themain factorswhendecidingwhere toplace aunit are: stabilityof ground;

access to sunlight and shading; exposure to wind and rain; availability of utilities; and availability of a greenhouse or shading structure.

•Therearethreemaintypesofaquaponics:themediabedmethod,alsoknownasparticulate bed; the nutrient film technique (NFT) method; and the deep water culture (DWC) method, also known as the raft method or floating system.


•Theessentialcomponentsforallaquaponicunitsare:thefishtank,themechanicaland biological filtration, the plant growing units (media beds, NFT pipes or DWC canals), and the water/air pumps.

•Themediabedsmust: (i)bemadeof strong inertmaterial; (ii)have adepthofabout 30 cm; (iii) be filled with media containing a high surface area; (iv) provide adequate mechanical and biological filtration; (v) provide separate zones for different organisms to grow; and (vi) be sufficiently wetted through flood-and-drain or other irrigation techniques to ensure good filtration.

•ForNFTandDWCunits,mechanicalandbiofiltrationcomponentsarenecessaryin order to respectively remove the suspended solids and oxidize the dissolved wastes (ammonia to nitrate).

•ForNFTunits,theflowrateforeachgrowpipeshouldbe1–2 litres/minutetoensure good plant growth.

•ForDWCunitseachcanalshouldhavearetentiontimeof2–4 hours.•HighDOconcentrationisessentialtosecuregoodfish,plantandbacteriagrowth.

In the fish tank DO is supplied by means of air stones. Media bed units have an interface between the wet zone and dry zone that provides a high availability of atmospheric oxygen. In NFT units, additional aeration is provided into the biofilter, while in DWC air stones are positioned in both biofilter and plant canals.

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4. Design of aquaponic units · 2021. 4. 13. · 36 Small-scale aquaponic food production – Integrated fish and plant farming FIGURE 4.3 Taro (Colocasia esculenta) plants growing